The present invention relates to a method for controlling the downhill speed of an off-the-road vehicle, and more particularly to a method by which the vehicle speed can be set to a constant descending speed by an electronic system operating a braking system with active brake intervention within a low-range traveling phase.
Known devices for maintaining a constant speed desired by the driver through automatic application of the brakes, a process referred to as "active braking," are already provided in road vehicles. As a general rule, such vehicles are equipped with an anti-locking brake system (ABS), as well as with an anti-slip regulation (ASR) system. The operating components already present in such systems, such as, the electronics, wheel speed sensors, solenoid control valves, etc., are used also to carry out the above-mentioned speed regulation. Such known active braking devices relieve the driver of the task of having to constantly actuate the brake pedal during travel on a downhill stretch. In addition, a uniform speed is evenly maintained in a reliable manner over varying gradients.
An electrical/pneumatic braking force regulating circuit is known, for example, from German application DE-A-24 19 242. In accordance with such circuit, the vehicle speed while traveling on a gradient is maintained by active braking initiated when neither the gas nor the brake pedal is actuated. Such speed regulation is terminated in immediate response to actuation of any one of the above-mentioned pedals, or when the superimposed ABS becomes active.
Another known method is disclosed, for example, in WO 96/11826 A1, in which a vehicle is equipped with a system for downhill control, also referred to as "hill descent control" (HDC). The control system disclosed therein can be activated by the driver by means of a switch. When activated, such system maintains the vehicle on a steep gradient at a constant low speed by means of active, controlled braking, thereby obviating actuation of the brakes by the driver. This system is particularly intended for off-the-road vehicles traveling on a gradient that is so steep that the engine braking effect is no longer sufficient to decelerate the vehicle, even in the lowest gear.
Finally, a process for controlling the speed of a vehicle on a descending gradient is disclosed in German patent DE 196 48 559 (corresponding to Ser. No. 08/970,988 of the inventors herein), whereby the vehicle speed can be set to a constant downhill speed by means of an electronic system through active braking intervention. In accordance with the process, a continuously adjustable target speed to which the current vehicle speed is set is synthesized within the electronic system to achieve a comfortable transition from the vehicle speed and the constant downhill speed, resulting in a continuous transition to the constant downhill speed.
It is the object of the present invention to provide a method for controlling downhill speed by means of which active, controlled braking of an off-the-road vehicle with several gears takes place in an especially comfortable manner in a low-range traveling speed range.